In the process of electrophotography, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles commonly referred to in the art as toner. By methods now well known in the art, the residual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto.
Problems associated with transferring the latent image to a support, especially the problem referred to as "toner offset", have been well documented in the field. Methods of overcoming the problem have also been documented. For example, U.S. Pat. No. 4,264,181 to Lentz et al., U.S. Pat. No. 4,257,699 to Lentz and U.S. Pat. No. 4,272,179 to Seanor, all commonly assigned to Xerox, describe fuser members and methods of fusing thermoplastic resin toner images to a substrate wherein a polymeric release agent having functional groups is applied to the surface of the fuser member.
While the mechanism involved is not completely understood, it has been observed that when certain polymeric fluids having functional groups are applied to the surface of a fusing member having an elastomer surface with a metal oxide, metal salt, metal, metal alloy or other suitable metal compounds dispersed therein there is an interaction (a chemical reaction, coordination complex, hydrogen bonding or other mechanism) between the metal of the filler in the elastomer and the polymeric fluid having functional groups so that the polymeric release agent having functional groups in the form of a liquid or fluid provides an excellent surface for release which have an excellent propensity to remain upon the surface of the fuser member. Regardless of the mechanism, there appears to be the formation of a film upon the elastomer surface which differs from the composition of the elastomer and the composition of the polymeric release agent having functional groups. This film, however, has a greater affinity for the elastomer containing a metal compound than the toner and thereby provides an excellent release coating upon the elastomer surface. The release coating has a cohesive force which is less than the adhesive forces between heated toner and the substrate to which it is applied and the cohesive forces of the toner. The interaction between the functional group of the polymeric release agent and the metal of the elastomer containing metal leads to an overall diminution of the critical or high surface energy of the metal in the metal containing filler.
According to techniques known in the art, the toner release agents may be applied to the fuser roll by several delivery mechanisms including wicking, impregnating webs and by way of a donor roll which may comprise a high temperature vulcanized silicone rubber material.
While these silicone elastomer donor rolls have been commercially successful in some commercial applications they suffer from certain difficulties in that they tend to swell from being in contact with a silicone oil release agent which migrates or is absorbed into the silicone rubber. While a small degree of swelling may be acceptable if it is uniform, failure of such rolls has been observed by excessive swelling over a period of operation wherein the donor roll may actually be twice the original size. Under such circumstances, the silicone rubber donor roll may no longer function in providing a uniform layer of release fluid to the fuser roll.
Further, while donor rolls such as those described in U.S. Pat. No. 4,659,621 have attractive oil delivery capabilities in that they are capable of transporting sufficient quantities of functional release agent to the fuser roll to form the interfacial barrier layer between the fuser roll and the toner, they also tend to swell with the oil penetrating the rubber whereby there may be an interchange of the siloxane oil with the siloxane in the silicone rubber network leading to breakdown of the network and a lower crosslinked network. This reduces the toughness of the silicone rubber barrier layer as more release agent penetrates the surface. This difficulty is particularly pronounced when operating at temperatures in excess of 300.degree. F. Another failure mode is referred to as debonding wherein the swelling of the silicone rubber becomes so significant that it actually delaminates from the core of the donor roll.
Another recent development described in U.S. Pat. No. 5,061,965 to Ferguson et al. describes the use of a donor roll made of a base member, an intermediate comformable silicone elastomer layer, and an elastomer release layer comprising poly(vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene) where the vinylidene fluoride is present in an amount &lt;40 mole %, a metal oxide present in an amount sufficient to interact with polymeric release agent having functional groups to transport a sufficient amount of polymeric release agent to provide an interfacial barrier layer between the fusing surface and the toner. This donor roller suffers from the oil wetting capability between nonfunctional PDMS release agent and the nonreactive donor roller surface, since the invention counts on the polymeric release agent having functional groups to react with the metal oxide which is dispersed in the fluoroelastomer layer.
It would be desirable to have further improvement in the field to overcome the problems of toner offset and donor roll durability.